Platelet-Endothelial Cell Adhesion Molecule-1 (PECAM-1, CD31, endoCAM) is a member of the immunoglobulin (Ig) superfamily present on platelets, endothelial cells and leukocytes. Despite recent data implicating PECAM-1 in angiogenesis, cardiovascular development and inflammation much remains to be learned about the adhesive interactions of PECAM-1 that underlie these important processes. Therefore the overall goal of this proposal is to advance our understanding of the molecular basis of PECAM-1-mediated adhesion. Recent published and preliminary data have suggested that 1) there are specific regions of the cytoplasmic domain that regulate not only the ability of PECAM-1 to support adhesion, but that define its ligand specificity and 2) a distinct region in the extracellular domain may mediate PECAM-1 homophilic adhesion (PECAM-1 to PECAM-1). To test these hypotheses, the following specific aims are proposed. 1. Characterization of the functional regions of the cytoplasmic domain that regulate the adhesive function of PECAM-1. To accomplish this, a series of mutants bearing deletions of specific regions of the cytoplasmic domain will be constructed and expressed in L-cell fibroblasts for functional analysis in an aggregation assay. The characteristics of L-cell aggregation mediated by each mutant will be determined and the mechanisms by which the cytoplasmic domain regulate PECAM-1-dependent adhesion will be studied. 2. Characterization of the homophilic binding domain of PECAM-1. To accomplish this, a series of mouse/human PECAM-1 chimeric receptors will be expressed in L-cells and subsequently tested for their ability to participate in homophilic interactions. Confirmation of the functional significance of the sequence(s) determined to be required for PECAM-1 homophilic binding will be achieved by studying the effect of peptides that mimic these residues in in vivo models of angiogenesis and inflammation. Not only will our studies provide specific informaiton about the structure- function relationships of PECAM-1, (information that may prove helpful in the development of new therapeutic agents for inflammation and cancer), we believe that they will also advance our understanding of the basic mechanisms of cell-cell adhesion.